Enping Zhou

The nature of the central engine powering short gamma-ray bursts (sGRBs) in binary neutron star (BNS) mergers remains a key open question in the era of multi-messenger astronomy. The ergostar hypothesis, that a rapidly rotating star with an ergoregion can act as a powerful energy source, offers an alternative explanation to the black hole-accretion disk paradigm. In this work, however, we examine this hypothesis using a phenomenological EOS of strangeon matter, i.e., condensed matter with nucleon-like units for three flavors of quarks. By constructing a large suite of uniformly rotating equilibrium models, we systematically investigate the parameter space of the stable ergostars and calculate their maximum extractable energy. We demonstrate that strangeon matter supports a vast and robust parameter space for dynamically stable ergostars, even without requiring differential rotation. We find that the extractable rotational energy from these configurations can be on the order of 0.01 M _⊙ , representing a massive energy reservoir, even when accounting for baryonic mass variations (e.g., mass ejection and particle capture during the Penrose process). Our results suggest that BNS merger remnants composed of exotic matter could play a crucial, previously underestimated role in high-energy astrophysics.

We calculate the $f$-mode frequency and tidal overlap of quark stars using the full general relativity method. We verify the universal relations obtained from conventional neutron stars in the case of quark stars and explore the cases with different values of parameters of the quark star equation of state. Since quark stars have significantly smaller radii compared to neutron stars in the low mass range, the relation between the tidal defomability and $f$-mode frequency times radius is different for neutron stars and quark stars. This difference has an impact on dynamical tide, which is the lowest-order effect we know of that can distinguish quark stars and neutron stars from the gravitational wave during the inspiral phase. We calculate the tidal dephasing caused by this effect in the post-Newtonian method and find that it can not be detected even by the next-generation gravitational wave detectors.

The investigation of the phase state of dense matter is hindered by complications of first-principle nonperturbative quantum chromodynamics. By performing the first consistent general-relativistic calculations of tidal-excited g-mode of neutron stars with a first-order strong interaction phase transition in the high-density core, we demonstrate that gravitational wave signal during binary neutron star inspiral probes their innermost hadron-quark transition and provides potent constraints from present and future gravitational-wave detectors.

The investigation of the phase state of dense matter is hindered by complications of first-principle nonperturbative quantum chromodynamics. By performing the first consistent general-relativistic calculations of tidal-excited g-mode of neutron stars with a first-order strong interaction phase transition in the high-density core, we demonstrate that the gravitational-wave signal during binary neutron star inspiral probes their innermost hadron-quark transition and provides potent constraints from present and future gravitational-wave detectors.

The equilibrium configuration of a solid strange star in the final inspiral phase with another compact object is generally discussed, and the starquake-related issue is revisited, for a special purpose to understand the precursor emission of binary compact star merger events (e.g., that of GRB211211A). As the binary system inspirals inward due to gravitational wave radiation, the ellipticity of the solid strangeon star increases due to the growing tidal field of its compact companion. Elastic energy is hence accumulated during the inspiral stage which might trigger a starquake before the merger when the energy exceeds a critical value. The energy released during such starquakes is calculated and compared to the precursor observation of GRB211211A. The result shows that the energy might be insufficient for binary strangeon-star case unless the entire solid strangeon star shatters, and hence favors a black hole-strangeon star scenario for GRB211211A. The timescale of the precursor as well as the frequency of the observed quasi-periodic-oscillation have also been discussed in the starquake model.

Can pulsar-like compact objects release further huge free energy besides the kinematic energy of rotation? This is actually relevant to the equation of state of cold supra-nuclear matter, which is still under hot debate. Enormous energy is surely needed to understand various observations, such as γ-ray bursts, fast radio bursts and soft γ-ray repeaters. In this paper, the elastic/gravitational free energy of solid strangeon stars is revisited for strangeon stars, with two anisotropic models to calculate in general relativity. It is found that huge free energy (>10⁴⁶ erg) could be released via starquakes, given an extremely small anisotropy ((p _t − p _r)/p _r ∼ 10⁻⁴, with p _t/p _r the tangential/radial pressure), implying that pulsar-like stars could have great potential of free energy release without extremely strong magnetic fields in the solid strangeon star model.